Reduction of transmission loss in bridged subscriber loops



Feb. 9, 1960 L. HocHGRAF REDUCTION oF TRANSMISSION Loss IN BRIDGESSUBSCRIBER Loops Filed Aug. 1s, 1957 /NVEA/ron L. HOCHGRAF @y l Arron/Er.will 'dalla nals.

nnnUcrroN or TRANSMISSION Loss 1N nnrnenn spscnn Loors Lester Hochgraf,Madison, NJ., assignor to Bell Telephone Laboratories,Incorporated, NewYork, NX., a corporation of New York This invention relates toimprovements in communication systems and more particularly to thereduction of transmission losses in multiloop transmission systems.

It often happens in the design ory installation of communication systemsthat it is desirable to connect two or more communication channels inmultiple. Thisris particularly true in telephone systems where, inrendering certain services, itis common practice to bridge subscriberloops or wire pairs across a common transmission medium. This medium maybe, for example, a common line terminating in a central ottico, orterminal equipment of channels for the transmissionor reception of sigThe need for this expedient arises, for example, where multiparty linesare provided or where so-called secretarial services are given. Further,it is quite comlmon for professional men who have business lines intheir oflices to` have extensions of these lines located at theirresidences` which'may beat some distance from their oiices andfrequently in another exchange area.

One problem which results `from this practice of multiply-connectingsubscriber loops is that of overcoming v the increase in transmissionloss caused by the combined open-circuited or idle. Moreover, thisproblem becomes more pronounced with increasing loop length.

Accordingly, it is an object of the present invention to improve signaltransmission incommunication systems khaving multiply-connectedtransmission paths.

More specifically, it is an object of this invention to reducesubstantially transmission losses in telephone systems where two or morewire pairs are connected in multiple. v The use of relatively `expensiveand complicated relay circuits to improve speech transmission intelephone systems having multiply-connected subscriber loops has beenknownin the art. Such circuits are commonly called bridge litters. Asrepresentative of this practice, U.S. Patent No. 2,041,879, which issuedMay 26, 1.936, to H. W. Ulrich, is noted. The relay circuits, showntherein to eect an improvement in speech transmission requireconsiderable auxiliary circuitry, are relatively costly and detailed,and present maintenance problems inherent in devices having moving partsand contacts. As a result, these relay circuits have been used withconsiderable discretion, and only when such improvement in speechtransmission has been found mandatory. Wherever posv tional requirementthat there not be any bridged loops 2,924,667 Patented Feb. 9, 1960 icemake more economical and simple the means by which the reduction oftransmission losses in telephone systems having bridged subscriber loopsis eected. It is a related object to make such means small and rugged sothat a minimum of maintenance and protection is required.

In an illustrative embodiment'of the present invention, described inmore detail below, bridged-tap losses as they are sometimes known, aregreatly reduced. This is accomplished by connecting saturable corereactors in series with each multipled lo-op near the point at which theloop is bridged on the common line. The impedances of these reactorsdepend on loop direct current and are such as to introduce a negligibleloss into active loops but a very high loss into idle loops. Idle loopsthus appear to the common line as substantially open circuits so thattheir attendant shunt capacity does not result in a bridged loss on anactive loop.

It is a well-known property of the saturable core reactor that it has arelatively high inductance when its core is unsaturated, and arelatively low inductance when its core is saturated. That value ofcurrent which is necessary to cause this saturated condition is known asthe saturation current. Further reference as to the nature andproperties of saturable core reactors (or saturable core inductors asthey are often called) may be made to the following literature: G. W.Elman, Magnetic Alloys of Iron, Nickel, and Cobalt, 54 ElectricalEngineering 1292 (December 1935); and E. Peterson, Coil Pulsers forRadar, 25 Bell System Technical Journal 603 1(October 1946).

A fuller understanding of the nature of the invention and of its variousobjects, features, and advantages may be had from a consideration of anillustrative embodiment now to be described with reference to theaccompanying partially schematic drawing which shows an` illustrativesystem in accordance with the invention in which a plurality ofsubscriber loops are multiply-connected to a common line terminating inacentral office.

In the drawing, two relatively long subscriber loops L1 and L2, theterminations of which are located at some distance from each other andfrom their associated central otlice 10, are connected in multiple atthe central office to common terminals 12 and 14 by way of a common lineL3. 'i

For the sake of simplicity, only two subscriber loops are shown.Likewise, wherever possible, the well-known particulars of the centraloice and those of substations S1 and S2 have been omitted. For thecircuit details of a typical central office and substation, reference ismade to Patent No. 2,585,904, which issued February 19, 1952, to A. I.Busch and to Patent No. 2,629,783, which issued February 24.V 1953, toI-I. F. Hopkins, respectively.

Connected in series with loop conductors 16 and 18 of subscriber loop131 are windings 20 and 22, respectively, of saturable core reactor No.I. These windings are balanced-wound on a saturable core 24 to beseriesaiding to loop (metallic) currents, as shown by the polaritymarkings. It follows, therefore, that these windings are wound inparallel-opposing relation to longitudinal currents. That the windingsare balanced-wound should be `understood to mean that they each havesubstantially the same impedance, have substantially the same number ofturns, and are symmetrically disposed on core 24. This balancing of thewindings is done to l 3 known in the art andare taught, for example, inPatent 1 Nu. 2,326,053 tu C. M. Morris, which issued August 3, 1943. Itcan be seen that the magnetic effects in core 24 or longitudinalcurrents induced in conductors 16 and 18 W1ll cancel out, therebyavoiding the magnetization of core 24 by these currents. Similarly, thewindings 26 and 28 ofsaturable core reactor No. II are serially insertedinloop conductors 30 and 32, respectively, of subscriber loop L2y andare balanced-wound on core 34- 1n series-aiding relation to loopcurrents. The magnetic paths of saturable cores 24 and 34 may havetolerable dlscontinuities (air gaps) as, for example, in the case of atape-wound core, or they may have continuous magnetic paths,everywherevhaving a permeability greater than one as, for example, inthe case of a laminated disc-type core.

Control circuit 36 serves the purpose of answering or extending calls onthe various subscriber loops. The ringing generator 38 serves thepurpose of ringing the various substations and, by operation of key 40,is connected via conductors 42 and 44, control circuit 36, and commonline L3 to any substation to be rung.

Various combinations of calls are possible in the system shown. Forexample, a subscriber may wish to call another subscriber multipled tothe same common line. This is what is commonly known as a revertivecall. Illustrative of another type of call, the subscriber at substationS1 may wish to call or be called by another subscriber connected to anoutside line L1 terminating at Let it be assumed, therefore, that asubscriber (not,

shown) connected to line L4 desires to call the subscriber at substationS1. The proper connections are made in the control circuit 36, ringingkey 40 is operated, and ringing current is supplied by ringing generator38 to ringer '46 of substation S1. The ringing current is of sufficientamplitude to saturate any or all of the saturable cores and, therefore,the core 24 of saturable core reactor No. I becomes saturated so thatthe windings 20 and 22 present a relatively low impedance to ringingcurrents passing therethrough. Likewise, if the ringing circuitassociated with substation S2 is designed .to respond to the same typeof ringing signal to which substation S1 responds, the ringing currentin loop L2, in owing through windings 26 and 28, will saturate core 34and thereby ring substation S2. i

When substation S1 responds to the call, i.e., 'goes olfhook, loop L1-is said to be active and a direct-current path is completed in awell-known manner from the central oiice battery 48, through theswitchhook contacts 50 and the remainder of the substation directcurrent circuit, shown here simply as an effective impedance Zs1. Theloop direct current supplied by battery 48 flows through windings 20 and22y of reactor No. I and is sufficient in magnitude to saturate core 24.When saturated, reactor No. I presents a negligible impedance (e.g.,less than 100 ohms at one kilocycle per second) to speech currents onthe loop.

If substation S2 remains on-hoo'k, i.e., idle, no directcurrent pathfrom battery 48 is completed in loop L2 and core 34 remains unsaturated.Reactor No. II thus presents a relatively high impedance (e'.g., morethan 20,000 ohms at one kilocycle per second) to speech cur rents, andeffectively removes the stray capacity CS2 as a bridging loss on activeloop L1. Were it not for reactor No. II, loop L2, by virtue of its straycapacity CS2, would introduce a substantial transmission loss intoactive loop L1 even though substation S2 is olf-hook and is an opencircuit for direct current. This loss, however, is avoided by theprovision of a saturable reactor, as shown, fin physical andelectricalproximity to the bridging point 'yon-line L3.

s Y s.

loop L2 when loop L2 is active and loop lL1 idle. The same principlesare applicable in an obvious manner where more than two loops arebridged on the line. Of course, where several loops are active at once,as, for example, where substation S1 is in a vrevertive talkingconnection with substation S2, it is necessary that the common battery48 supply su'cient current to saturate the reactors of all vactivecircuits. Or, stated otherwise, itis necessary in such a situation thatthe reactors have sufficiently low saturation currents to achieve a lowimpedance condition when their associated loops are active. However, itis to be understood that these saturation currents should not be so lowthat speech currents alone will suce to saturate the reactors. Thiswould render the reactors ineiective as bridgeV lifters, for when theirassociated loops are idle, they must otter a high impedance to speechcurrents.

The saturable core reactor meets the requirement that it worksubstantially equally well whether metallic or grounded ringing isprovided in the various loops of the system. This result is achieved bymaking the ringing current in one of the loop conductors alone ofsuftcient amplitude to cause saturation of the core associatedwith theloop. Metallic ringing, of course, is accomplished by shunting theringer of a substation across its associated loop, as is shown in thedrawing; whereas grounded ringing is accomplished by grounding one sideof the ringerl and connecting the other side to one of the loopconductors. Thus, in the case of metallic ringing of, for example,substation S1,` ringing current would ow through both of the conductors16 and 1S and therefore both of the windings 20 and 22; whereas in thecase of grounded ringing of substation S1, ringer 45 would be connectedAbetween ground and one of conductors 16 and'lS and ringing current wouldflow only through that one of these conductors and its associatedwinding.

Also, dialing pulses, which, for example, may be generated byinterrupting the current from a direct-current source, are substantiallyunimpaired by the insertion of saturable core reactors in subscriberloops.

Thus, each of the reactors may be considered as a level dilerentiationdevice, presenting ahigh impedance to voice signals when it is in anunsaturated state, and a low impedance both to voice signals when it issaturated by common battery current and to ringing signals in theabsence of common battery current.`

Although the present invention has been described with reference of aspeciiic embodiment, it should be considered as illustrative, forv theinvention comprehends also such other embodiments as come within itsspirit and scope.

What is claimed is:

l. A communication system comprising a communication center, at leastone arterial line connected to said center for the transmission ofsignals, a plurality of transmission lines multiply-connected to saidarterial line and each terminated in means for the transmission andreception of said signals, a saturable core reactor individual to andconnected in the path of signal current flow in each of saidtransmission lines, and means for saturating said reactors, saidreactors each presenting a relatively high impedance to signal currentswhen unsaturated and a relatively low impedance to signal currents whensaturated.

2. In a signal current communication system, a source of direct current,a common transmission medium for the transmission of said signalcurrents, a plurality of transmission lines each connected in multipleto said common transmission medium, means individual to and associated`with each of said lines for completing a circuit therethrough for thetransmission of direct current froml saidsource, a plurality of.saturable core reactors,

current being of sufficient magnitude lto saturate all of said reactors,each of said reactors exhibiting a relatively high impedance to saidsignal currents when its associated line is enabled for the transmissionof direct current and a relatively low impedance to said signal currentswhen its associated line' is disabled for the transmission of directcurrent. n

3. A communication system in accordance with claim 2 wherein each ofsaid saturable core reactors is proximately-connected to saidtransmission medium.

4. In a telephone system, a central oiiice including a source of ringingsignals, a plurality of subscriber loops each including a substationcomprising signalling means responsive to said ringing signals, a commontransmission medium multiply-connected to said loops, means connectingsaid medium to said central o'ice, and a reactor in each of saidsubscriber loops having a saturable core adapted to be saturated inresponse to said ringing signals.

5. A telephone system in accordance with claim 4 wherein said loops eachcomprise a pair of conductors and wherein each of said reactorscomprises two windings wound on its associated saturable core, one ofsaid windings being serially connected in one of the conductors of itsassociated subscriber loop and the other Winding being seriallyconnected in the other conductor of said associated subscriber loop.

6. A system in accordance with claim 5 wherein said windings are woundon said core in series-aiding relation to loop currents.

7. A telephone system in accordance with claim 6 in which each of saidwindings has substantially the same impedance, whereby the magneticcondition of each of said saturable cores is substantially unaffected bylongitudinal currents in its associated subscriber loop.

8. In a telephone system, a plurality of normally open-circuitedsubscriber loops each comprising a pair of conductors, a reactor, and aterminating substation, said reactor having a saturable core and an evennumber of windings serially inserted in said pair of conductors andbalanced-wound on said core in series-aiding relation to loop currents,said core saturating at a predetermined value of current through saidwindings, said terminating substation comprising means for completing acircuit through its associated loop, a common transmission mediumconnected in multiple to said loops and terminating in a central oficecomprising a source of direct current common to said subscriber loops,said direct current having a value equal to or greater than saidpredetermined value of saturation current.

9. A telephone system in accordance with claim 8 wherein said centralolice comprises ringing means for generating and distributing ringingcurrents to any of said loops of a magnitude sucient to saturate any ofsaid cores.

10. In a telephone system, a plurality of relatively long normallydisabled subscriber loops each terminated at one end in a signal energyterminal, a longitudinallybalanced saturable core reactor comprising asaturable core associated with each of said loops and connected near theother end of its associated loop in the path of signal energy flow,individual means for enabling or disabling each of said loops for thetransmission of direct current, and a common transmission mediumconnected in multiple to the said other ends of said loops, a centraloffice including a source of direct current of suflicient magnitude tosaturate the core associated with any of said loops, means forconnecting said source to said medium, said central oliice furthercomprising means for selectively routing signals to and from saidterminals, cach of said reactors presenting `a relatively highimpedanceto said signal energy when its associated loop is disabled forthe transmission of direct current, thereby substantially eliminatingthe loss in signal transmission through enabled loops caused by thecombined shunt capacity of disabled loops, and presenting a relativelylow impedance to said signal energy when its associated loop is enabled.

11. A telephone system in accordance with claim 10 wherein said centraloice further comprises ringing means for generating and selectivelyrouting ringing signals to any of said terminals, said ringing signalsbeing of sufiicient amplitude to saturate the core associated with anyof said loops.

12. In combination, a first transmission line, a plu-l rality of furthertransmission lines each connected to said first line and each terminatedin an associated signal translating device, a source of direct currentconnected to said first transmission line, means for enabling each ofsaid signal translating devices, means responsive to the enabling ofeach of said signal translating devices for completing a direct-currentpath from said source of direct current through the one of said furthertransmission lines associated with each of said signal translatingdevices, and a saturable core reactor connected in series with each ofsaid further transmission lines in physical proximity to said rsttransmission line and relatively remote from its associated signaltranslating device, said reactors each being adapted to saturate inresponse to direct current owing from said source.

13. In combination, a central oliice including a source of directcurrent, a plurality of telephone sets remote from said central oflice,each of said sets being adapted to receive and transmit speech signalsin an oil-hook condition and each being disabled for the transmissionand reception of speech signals in an on-hook condition, said sets alsobeing adapted to complete a directcurrent path including said source ofdirect current only in said oli-hook condition, a transmission lineextending from said central oice, transmission loops for each of saidsets bridged across said line, each of said loops being characterized bysufficient stray capacity to introduce a substantial transmission lossinto any loop whose associated set is oli-hook, means for reducing saidloss comprising a plurality of saturable core reactors one individual toand associated with each of said loops, and means connecting each ofsaid reactors in series with its associated loop in physical proximityto said Itransmission line, each of said reactors being in anunsaturated condition and presenting a high impedance to speech signalsin the absence of direct-current flow in its associated loop from saidsource, the impedance of each of said reactors to speech signals whensaturated being relatively low.

14. A combination in accordance with claim 13 wherein each of saidreactors is longitudinally-balanced.

15. A combination in accordance with claim 13 wherein said centraloliice further includes a source of ringing current for alerting any ofsaid telephone sets, said current being sucient in amplitude to saturateany of said reactors.

16. A combination in `accordance with claim 15 wherein said plurality oftelephone sets comprises a pair of sets and wherein the magnitude ofsaid direct current and the, amplitude of said ringing current are eachsutcient to simultaneously saturate the reactors associated with saidpair of sets.

References Cited in the le of this patent UNITED STATES PATENTS1,266,158 Reid May 14, 1918 2,192,061 Woodworth Feb. 27, 1940 2,332,043`Atkins Oct. 19, 1943 Notice of Adverse Decision in Interference InInterference No. 91,473 involving Patent No. 2,924,667 L. Hochgraf,Reduction of transmission loss in bridged subscriber loops, finaljudgment ndered Sept. 18, 1962, as to claim 1.

adverse to the patentee was re [Oez'al Gazette October 16', 1962.]

